hybridgroup.gobot/system/spi_gpio.go

package system

import (
	"fmt"
	"time"

	"github.com/hashicorp/go-multierror"
	"gobot.io/x/gobot/v2"
)

type spiGpioConfig struct {
	pinProvider gobot.DigitalPinnerProvider
	sclkPinID   string
	nssPinID    string
	mosiPinID   string
	misoPinID   string
}

// spiGpio is the implementation of the SPI interface using GPIO's.
type spiGpio struct {
	cfg spiGpioConfig
	// time between clock edges (i.e. half the cycle time)
	tclk    time.Duration
	sclkPin gobot.DigitalPinner
	nssPin  gobot.DigitalPinner
	mosiPin gobot.DigitalPinner
	misoPin gobot.DigitalPinner
}

// newSpiGpio creates and returns a new SPI connection based on given GPIO's.
func newSpiGpio(cfg spiGpioConfig, maxSpeed int64) (*spiGpio, error) {
	spi := &spiGpio{cfg: cfg}
	spi.initializeTime(maxSpeed)
	return spi, spi.initializeGpios()
}

func (s *spiGpio) initializeTime(maxSpeed int64) {
	// maxSpeed is given in Hz, tclk is half the cycle time, tclk=1/(2*f), tclk[ns]=1 000 000 000/(2*maxSpeed)
	// but with gpio's a speed of more than ~15kHz is most likely not possible, so we limit to 10kHz
	if maxSpeed > 10000 {
		if systemDebug {
			fmt.Printf("reduce SPI speed for GPIO usage to 10Khz")
		}
		maxSpeed = 10000
	}
	tclk := time.Duration(1000000000/2/maxSpeed) * time.Nanosecond
	if systemDebug {
		fmt.Println("clk", tclk)
	}
}

// TxRx uses the SPI device to send/receive data. Implements gobot.SpiSystemDevicer.
func (s *spiGpio) TxRx(tx []byte, rx []byte) error {
	var doRx bool
	if rx != nil {
		doRx = true
		if len(tx) != len(rx) {
			return fmt.Errorf("length of tx (%d) must be the same as length of rx (%d)", len(tx), len(rx))
		}
	}

	if err := s.nssPin.Write(0); err != nil {
		return err
	}

	for idx, b := range tx {
		val, err := s.transferByte(b)
		if err != nil {
			return err
		}
		if doRx {
			rx[idx] = val
		}
	}

	return s.nssPin.Write(1)
}

// Close the SPI connection. Implements gobot.SpiSystemDevicer.
func (s *spiGpio) Close() error {
	var err error
	if s.sclkPin != nil {
		if e := s.sclkPin.Unexport(); e != nil {
			err = multierror.Append(err, e)
		}
	}
	if s.mosiPin != nil {
		if e := s.mosiPin.Unexport(); e != nil {
			err = multierror.Append(err, e)
		}
	}
	if s.misoPin != nil {
		if e := s.misoPin.Unexport(); e != nil {
			err = multierror.Append(err, e)
		}
	}
	if s.nssPin != nil {
		if e := s.nssPin.Unexport(); e != nil {
			err = multierror.Append(err, e)
		}
	}
	return err
}

func (cfg *spiGpioConfig) String() string {
	return fmt.Sprintf("sclk: %s, nss: %s, mosi: %s, miso: %s", cfg.sclkPinID, cfg.nssPinID, cfg.mosiPinID, cfg.misoPinID)
}

// transferByte simultaneously transmit and receive a byte
// polarity and phase are assumed to be both 0 (CPOL=0, CPHA=0), so:
// * input data is captured on rising edge of SCLK
// * output data is propagated on falling edge of SCLK
func (s *spiGpio) transferByte(txByte uint8) (uint8, error) {
	rxByte := uint8(0)
	bitMask := uint8(0x80) // start at MSBit

	for i := 0; i < 8; i++ {
		if err := s.mosiPin.Write(int(txByte & bitMask)); err != nil {
			return 0, err
		}

		time.Sleep(s.tclk)
		if err := s.sclkPin.Write(1); err != nil {
			return 0, err
		}

		v, err := s.misoPin.Read()
		if err != nil {
			return 0, err
		}
		if v != 0 {
			rxByte |= bitMask
		}

		time.Sleep(s.tclk)
		if err := s.sclkPin.Write(0); err != nil {
			return 0, err
		}

		bitMask = bitMask >> 1 // next lower bit
	}

	return rxByte, nil
}

func (s *spiGpio) initializeGpios() error {
	var err error
	// nss is an output, negated (currently not implemented at pin level)
	s.nssPin, err = s.cfg.pinProvider.DigitalPin(s.cfg.nssPinID)
	if err != nil {
		return err
	}
	if err := s.nssPin.ApplyOptions(WithPinDirectionOutput(1)); err != nil {
		return err
	}
	// sclk is an output, CPOL = 0
	s.sclkPin, err = s.cfg.pinProvider.DigitalPin(s.cfg.sclkPinID)
	if err != nil {
		return err
	}
	if err := s.sclkPin.ApplyOptions(WithPinDirectionOutput(0)); err != nil {
		return err
	}
	// miso is an input
	s.misoPin, err = s.cfg.pinProvider.DigitalPin(s.cfg.misoPinID)
	if err != nil {
		return err
	}
	// mosi is an output
	s.mosiPin, err = s.cfg.pinProvider.DigitalPin(s.cfg.mosiPinID)
	if err != nil {
		return err
	}
	return s.mosiPin.ApplyOptions(WithPinDirectionOutput(0))
}